With 1.5 million acres planted annually, wheat is the fourth largest crop in Minnesota’s agriculture. When diseases and other crop pests are not limiting, average yields continue to increase steadily. Adequate and efficient use of fertilizer is a major contributor to this increase.
Nitrogen use generates the largest net return of any nutrient used in wheat production. It is important to use this nutrient as efficiently as possible.
In Minnesota, fertilizer N suggestions are determined using two strategies that depend on the area of the state in question. For the western portion of the state, where most of the wheat is grown, the soil nitrate test (soil samples collected to 2 feet) is the best and most accurate management tool for predicting the amount of fertilizer N to use. This soil test is recommended if wheat is grown in the shaded area of the state shown on the map.
If the soil nitrate test is used, the amount of fertilizer N required to meet the yield goal is calculated from the following equation:
NRec = [(2.5) x YG] - STN(0-24 in.) - NPC
If wheat is grown in the second year following any of the crops in Table 1 use the N credit listed in Table 2. For situations where the soil nitrate test is not used, suggestions for fertilizer N are based on a consideration of previous crop, yield goal and soil organic matter content.
The soil nitrate test is not used for making nitrogen fertilizer guidelines for wheat grown in eastern Minnesota (the area not shaded on the map). Nitrogen fertilizer guidelines for these situations are summarized in Table 3. Use the fertilizer N guidelines for soils having a high organic matter content when wheat is grown in southeast Minnesota. This recommendation applies in Goodhue, Wabasha, Olmsted, Winona, Fillmore and Houston counties.
Table 1: Suggested nitrogen credits for crops that might precede wheat in a crop rotation*| Previous crop | First year N credit lbs. N/acre |
|---|---|
| Soybean | 20 |
| Edible beans, field peas | 10 |
| Harvested sweet clover | 10 |
| Harvested alfalfa** or non-harvested sweet clover | |
| 4-5 plants/ft2 | 75 |
| 2-3 plants/ft2 | 50 |
| 1-2 plants/ft2 | 25 |
| 1 or fewer plants/ft2 | 0 |
| Harvested red clover | 35 |
| Sugar beet | |
| Yellow leaves at harvest | 0 |
| Light-green leaves at harvest | 15-30 |
| Dark-green leaves at harvest | 60-80 |
*Use these credits when the soil nitrate test is used.
**If the 3rd or 4th cutting of alfalfa was not harvested, add 20 lbs. N/acre to the N credits listed.
Table 2: Suggested nitrogen credits when wheat is grown two years after a legume crop| Previous legume crop | Second year N credit lbs. N/acre |
|---|---|
| Alfalfa (4+ plants/ft2), non-harvested sweet clover | 35 |
| Alfalfa (2-3 plants/ft2), Birdsfoot trefoil | 25 |
| Red clover | 20 |
The nitrogen supplied by legume crops can also be used by the wheat crop planted 2 years after the legume. The nitrogen credits for these situations are summarized in Table 2. Subtract these values from the N guidelines that are listed for crops in Group 2 (Table 3) when wheat is planted 2 years after a legume crop.
Nitrogen from the decomposing sugar beet tops can be used by the wheat crop following sugar beet. These N credits are based on the overall color of the sugar beet tops at harvest and are listed in Table 1. If the soil nitrate test is used for nitrogen guidelines, the value for the appropriate color should be used as the nitrogen credit from the previous crop (NPC) in the N recommendation equation. If the N guidelines are taken from Table 3, subtract the value for the appropriate leaf color in Table 1 from the appropriate N recommendation listed in Table 3.
Nitrogen guidelines, whether calculated from the equation or obtained from Table 3, should also be used for winter wheat production. For this crop, 15 to 30 lbs. N per acre should be applied in the fall either before or at the time of seeding. The remainder of the fertilizer N needed should be top-dressed early the following spring.
Research has shown that most of the total amount of nutrients used by wheat is absorbed from the soil between the growth stages of tillering and heading. So it is important to have an adequate supply of all nutrients in the root zone early in the growing season.
Since N is mobile in soils and can move to the roots with soil moisture, there is considerable flexibility in the management of this nutrient. For wheat production in most of Minnesota, fertilizer N can be applied in the fall after soil temperatures have stabilized at or below 50°F, or in the spring. In southeast, south-central, and central Minnesota, fertilizer N should be applied in the spring and incorporated before planting.
Because of the potential for losses due to leaching, fertilizer N should be applied in the spring when soils are sandy. Split N applications are encouraged for very sandy soils. For these situations, the first application can be made before planting followed by the remainder between tillering and jointing.
In northwest and western Minnesota, N applications at tillering may be justified if you suspect loss of previously applied N from leaching or denitrification. Applying N at this time would also be appropriate where a yield goal established in the fall was conservative and projected weather plus available soil moisture suggests there is a good probability for a higher yield. For these situations, added N at tillering may also increase the protein content of hard red spring wheat.
Table 3: Nitrogen fertilizer suggestions for wheat where the soil nitrate test is not used| Crop grown last year | Organic matter level* | Yield goal below 40 bu/acre | Yield goal 40-49 bu/acre | Yield goal 50-59 bu/acre | Yield goal 60-69 bu/acre | Yield goal 70-79 bu/acre | Yield goal 80+ bu/acre |
|---|---|---|---|---|---|---|---|
| Alfalfa (4+plants/ft2), non-harvested sweet clover | Low | 0 lbs N/acre | 0 lbs N/acre | 30 lbs N/acre | 55 lbs N/acre | 80 lbs N/acre | 95 lbs N/acre |
| Alfalfa (4+plants/ft2), non-harvested sweet clover | Medium/High | 0 | 0 | 0 | 35 | 60 | 75 |
| Soybeans | Low | 35 | 60 | 85 | 110 | 135 | 150 |
| Soybeans | Medium/High | 0 | 40 | 65 | 90 | 115 | 130 |
| Edible beans, field peas, harvested sweet clover | Low | 45 | 70 | 95 | 120 | 145 | 160 |
| Edible beans, field peas, harvested sweet clover | Medium/High | 25 | 50 | 75 | 100 | 125 | 140 |
| Any crop in Group 1 | Low | 0 | 30 | 55 | 80 | 105 | 120 |
| Any crop in Group 1 | Medium/High | 0 | 0 | 35 | 60 | 85 | 100 |
| Any crop in Group 2 | Low | 55 | 80 | 105 | 130 | 155 | 170 |
| Any crop in Group 2 | Medium/High | 35 | 60 | 85 | 110 | 135 | 150 |
| Organic soil | 0 | 0 | 0 | 0 | 30 | 35 |
* Low = less than 3.0%; Medium (Med.) and High = 3.0% or more.
Crops in Group 1: Alfalfa (2-3 plants/ft2), Alsike Clover, Birdsfoot Trefoil, Grass/legume hay, Grass/legume pasture, Fallow, Red Clover.
Crops in Group 2: Alfalfa (0-1 plants/ft2), Millet, Rye, Sweet corn, Barley, Corn, Mustard, Sorgum-sudan. Triticale, Buckwheat, Flax, Oat, Sugar beet, Wheat, Canola, Grass/hay/pasture, Potato, Sunflower, Vegetables.
If applied properly, all the common N fertilizers will have an equal effect on wheat yields. Some precautions in the application of some N sources are necessary. With anhydrous ammonia (82-0-0), there is concern for loss during application. If there is a strong ammonia odor emanating for the field, further application is discouraged since significant losses are likely occurring with application.
There is also a potential for N loss if urea (46-0-0) or urea-ammonium nitrate (28-0-0) is broadcast on the soil surface without incorporation when soil pH is higher than 7.3, air temperatures are 50 degrees F or greater and there is residue on the soil surface. Shallow incorporation of urea or fertilizers containing urea within 48 hours of application is encouraged when these N sources are used for wheat production. Fall application of 28-0-0 or 32-0-0 UAN is strongly discouraged due to a portion of the N in the fertilizer already in the nitrate form at application.
Late season applications of 30 lbs. N/ acre as Urea-ammonium nitrate solution (28-0-0) 2 to 5 days after anthesis (flowering) has been shown to increase hard red spring wheat grain protein 0.5 to 1.0%, 80% of the time. This rate will cause some leaf burn but will not harm grain yield. The economics of this application depend on the cost of the 28-0-0 and the protein premium of the wheat market. Do not apply during the heat of the day; instead apply either in early morning or evening application to limit the leaf burn by the 28-0-0. Do not tank mix N with any fungicides — wait 2 to 5 days after anthesis.
Suggestions for phosphate use are summarized in Table 4. The phosphorus (P) status of Minnesota soils is determined by using either the Bray or the Olsen analytical procedure. These tests are intended to be an index of crop response and not a direct measure of the amount of P in the soil. The Bray soil test uses a strong acid to extract P from the soil. In situations where carbonates are present in the soil, the acid in the Bray solution can be neutralized reducing the amount of P extracted and the effectiveness of the test. The Olsen test provides more accurate results if the soil pH is 7.4 or higher as it is not affected by carbonates in the soil. Both tests can be accurately used in situations where high soil pH is not an issue but values obtained from the Olsen tests will be lower for the same soil test classification range since the Bray and Olsen test extract P out of different pools of available P in the soil.
